1. Field of the Invention
The present invention relates generally to injection molding of foam products, and to gauging the size and shape of a mold cavity. Specifically, the present invention relates to gauging the size and shape of a mold cavity for injection molding components of articles of footwear.
2. Related Art
Foam products made by hot injection molding of certain material, such as olefin polymer-based foam, expand after manufacture. Once curing has taken place in the mold, the expansion occurs after the mold is opened. A manufacturer of any such product must therefore construct a mold whose cavity is smaller than the final desired size of the product. The problem facing the manufacturer is how to properly size, or gauge, the mold cavity. The manufacturer must determine how much smaller the mold cavity must be, relative to the desired size of the final product. The mold cavity must be gauged so that the product is the correct size after expansion and post-processing shrinkage.
Proper gauging is complicated by several factors. First, the amount of expansion, or expansion ratio, varies for different material. (Note: Expansion ratio is defined as the ratio of a dimension of an object (such as thickness) after expansion and post-processing shrinkage, to the same dimension before expansion.) Specifically, different proportions of a compound in combination with a blowing agent will have different expansion ratios.
Second, for many materials, a non-constant, non-linear relationship exists between expansion ratio and final thickness of a product at its thinnest point. (Note: Thickness of a mold cavity refers to the distance in the cavity between the two opposing mold surfaces. Thickness of a product refers to the corresponding distance on the product. The latter is illustrated in FIG. 8C, where the object depicted is a portion of the sole of a shoe.) For a specific material, a product will have expanded according to different expansion ratios to reach different thicknesses.
Manufacture of a product that is two centimeters thick, for example, requires a mold cavity of some lesser thickness. After removal from this mold, expansion takes place, yielding a final product that is two centimeters thick. Manufacture of a product that is one centimeter thick also requires a mold cavity of a lesser thickness. After removal from this mold, expansion takes place, yielding a final product that is one centimeter thick. But empirically, the expansion ratio in the first case is greater than in the second case. The two centimeter product will have undergone greater proportional expansion than the one centimeter product. In general, a relationship exists between expansion ratio and final thickness, such that the expansion ratio is greater with larger final thicknesses.
The relationship, however, is not constant and not linear. If the relationship between the expansion ratio and final thickness were linear or constant, it would be relatively easy to gauge a mold cavity. Knowing the final thickness of a product made with a given material would allow a straightforward determination of its expansion ratio. The necessary mold cavity size could then be determined based on the expansion ratio. Because the relationship between the expansion ratio and final thickness is neither linear nor constant, however, determination of expansion ratio given a final desired thickness is more problematic.
Third, if the final product is irregularly shaped, gauging the mold cavity becomes difficult. The sole for an item of athletic footwear, for example, is highly irregular. The thickness of the sole varies from point to point. Even if the expansion ratio can be determined for a specific thickness, gauging of a mold cavity for the sole becomes difficult because the expansion ratio will, like the thickness, vary from point to point.
Prior to the present invention, gauging of mold cavities has been done by trial and error. While ultimately effective, such an approach is wasteful, time consuming, and often results in misshaped or incorrectly sized products. Hence there is a need for a method by which mold cavities can be gauged accurately, so that the resulting product will be the necessary size and shape after expansion and post-processing shrinkage.
The present invention consists of a method for gauging a mold cavity to be used in the injection molding of a product, e.g., a sole or a portion of a sole of an article of footwear, made of material such as olefin polymer-based foam. The method comprises the steps of selecting a durometer for the product, determining a thickness expansion ratio based on the durometer, selecting a thickness value, selecting a material based on the thickness value and thickness expansion ratio, dividing a model of the final product into a plurality of sections, determining a length/width expansion ratio for each section, gauging a mold model for each section based on the expansion ratio for each section, and constructing a composite mold model using the gauged sectional mold models.